or those that don't check that a conf room is already booked and just show up expecting it to be empty. You still have that interruption "oh, I didn't know someone was using this room, sorry".
Unless I lock the door and/or hide in a corner, I've found that that interruption at least tends to resolve itself quickly and doesn't interrupt my flow. People react more predictably and reliably to familiar scenarios.
I've done that. It makes it worse. You get confused people jiggling the handle, knocking, and looking inside, as they try to enter the conference room they didn't book, after which you need to unlock the door, and explain to them that you locked the door to avoid interruptions, can they please go away and research the social signal a locked door is meant to convey.
The trick is you need to look unremarkable, yet also uninterruptible. I think the best strategy (short of finding an employer that values productivity) is to team up with someone else who values getting work done, and book a conference room with them. That way it looks like you're "having a meeting" and no-one will ask vapid questions.
Better still, dial your cell phone with the room's conference phone, so it looks like it's "on" to anyone peering through the door's window, and sit with your back to the door. People are less likely to try to take over the conference room if it looks like it's complicated to get you to vacate.